1. Field of the Invention
This invention relates generally to data bus arrangements and is more specifically related to data bus arrangements for use with Josephson junction devices. Still more specifically it relates to a star data bus interconnection arrangement whereby a plurality of ports are placed into two-way communication with each other without the generation of spurious, reflected signals. Each of the Josephson devices which is controlled by outputs from a computer or other logic circuit arrangement on an associated control line is supplied from its own constant current source. Each device is effectively placed in parallel with every other device by transmission lines of the same characteristic impedance connected at a node or nodes. The transmission lines which are terminated by a resistance equal to the characteristic impedance of the transmission line absorb incoming signals and prevent the reflection of such signals back to the node or nodes. The number of ports and transmission lines connected to a node is limited only by such practical considerations as the ability of a sensing junction to respond to the magnitude of the current pulse supplied to each transmission line after splitting at the node or nodes. In common with other terminated line Josephson circuits, switching speed in the order of tens of picoseconds are obtainable.
2. Description of the Prior Art
In known schemes, the outputs of logic circuits are connected to a separate sensing device which is responsive to the presence of current in the output circuit. All of the sensing devices are connected in series with each other, and to the extent there are outputs present, the associated sensing devices switch and a diminution of current is detected or, for a given time frame, the voltage drops may be summed to indicate that certain outputs are present. By using biases which strobe the sensing devices, in the latter instance, the particular output can be identified. One prior art arrangement which shows the summing of outputs is U.S. Pat. No. 3,458,735. The arrangement shown is, however, not a true data bus inasmuch as it is not a two-way data link between inputs and outputs. This should be clear from the fact that all the Josephson devices are in series and the outputs are summed to indicate how many of the outputs in the form of control currents are present. Each of the switchable devices is isolated and the signal due to switching of a given device is not coupled via transmission lines to any other device. The only possible coupling is via disturb signals which may cause spurious switching of adjacent devices.
To the extent that the definition of a data bus (a common interconnection which serves as a two-way data link between inputs and outputs of several logic blocks in a time shared manner) is fulfilled by the prior art, it is known to literally use a bus which is connected to a plurality of input and output devices which are all connected to the same bus. Such arrangements are common where the information being transmitted is in the form of variations in the voltage level applied to the bus. Thus, bipolar transistor circuits and field effect transistor circuits normally provide outputs in the form of voltages which can be applied to a bus with the assurance that all lines connected to the bus will see the same voltage. Under such circumstances, impedance matching, line lengths and proper terminations do not present severe design problems. In any event, this known data bus arrangement cannot be straightforwardly substituted in circuits where the information being transmitted is in the form of current variation. Impedance matching, signal reflection and proper termination become significant design problems because of the reality of current division which is a function of the number of stations or ports which must see the same current in parallel.
An arrangement which has data bus applications is shown in a co-pending application entitled "A Distributed Josephson Junction Logic Circuit", Ser. No. 636,868, filed Dec. 2, 1975, and assigned to the same assignee as the present invention. In that application, a number of Josephson devices are placed in series with a voltage source and a portion of the transmission line is utilized to actuate other devices. The transmission line is terminated at both ends in its characteristic impedance and is capable of bidirectional flow of data among a plurality of serially disposed actuable devices and control line portions.
An IBM Technical Disclosure Bulletin entitled "Josephson Junction Circuit " by G. J. Lasher, Vol. 11, No. 10, March 1969, p.1222, shows a Josephson junction in the transmission line environment which is terminated with the characteristic impedance of the strip line so that no reflected signal returns to the junction due to its own a.c. emission. This publication is concerned with improving the bistable behavior of a Josephson junction and is not concerned with a Josephson device which is connected in parallel with a number of other devices for communicating data therebetween.
IBM Technical Disclosure Bulletin, Vol.15, No.3, August 1972, p.899, in an article entitled "Josephson Junction Circuits Having Magnetic Feedback" by H. H. Zappe, shows in FIG. 5 a shift register arrangement which includes a pair of Josephson junctions connected in parallel, each of which is connected to the same power source and terminated at one end thereof in a resistance equal to Z.sub.o. A similar circuit is interleaved with the first pair of devices and each pair of Josephson devices is pulsed by alternate d.c. pulses applied to each pair. Each junction line acts as a control for the next junction. In this reference, there is no hint or suggestion that such an arrangement can be used as a data bus. Indeed, the use of the same source for pairs of devices indicates that pairs of devices are intended to be energized at the same time. This approach clearly obviates any possibility for developing a data bus in accordance with the teaching of the present application. The data bus arrangement of the present application is believed to be distinguishable and unobvious over all the references cited inasmuch as it takes advantage of the unique capability of a Josephson junction when it is in the zero voltage state to act, in combination with a series terminating resistance which matches the impedance of an associated transmission line, as a direct short to ground. In other words, the actuable device, when unactuated, has no internal impedance and, as a result, it is possible to utilize a transmission line connected to it, in one mode, as a driver and, in another mode, the same transmission line as a termination.